The Western Australian finance minister, Mike Nahan, will become the state’s new treasurer, replacing Troy Buswell who resigned on Monday following a car crash.

Premier Colin Barnett made the announcement on Friday morning as controversy surrounding the crash continued to plague his government.

Buswell quit cabinet this week after suffering a mental health breakdown that put him in hospital for 10 days in the wake of the crash in the early hours of 23 February.

Nahan will not have long to ease into the job with the state’s budget due in May.

The Riverton MP will retain the energy and citizenship and multicultural portfolios.

Dean Nalder, the MP for Alfred Cove, will pick up Buswell’s transport portfolio and finance from Nahan.

The premier said Nahan would make an “outstanding treasurer”.

“Mike Nahan has proven an excellent member of cabinet and has impressed me with his ability to work through complex financial and logistical matters in the energy and finance portfolios over the last year,” he said. “I look forward to working closely with him.”

Barnett thanked Buswell for his enormous contribution to the state and wished him well.

“Troy served the Liberal National government very well and we will miss his intellect and his capacity for hard work,” he said.

Meanwhile, the WA police commissioner, Karl O’Callaghan, said on Thursday he still had not spoken to Buswell.

O’Callaghan said forensic work was still to be done on the vehicle and police had not yet established who was driving.

The state opposition demanded a public inquiry into the crash. WA Labor leader Mark McGowan said he wanted the investigation to establish whether the government had covered up any criminality.

The premier has repeatedly said the first he heard about the car accident was when Buswell’s chief of staff, Rachel Turnseck, told him on Sunday 9 March. Buswell resigned the next day.